The present invention relates to a structured surface, having a support layer and, connected electrically to it, peak-shaped elements. The invention also relates to the use of this structured surface and a process for its manufacture.
All components in the field of vacuum electronics, such as cathode ray tubes for example, require a cathode to emit electrons into the vacuum. Up to now mainly thermal cathodes have been used for this purpose. These cathodes are heated to temperatures of 1000.degree. C. and more in order that the electrons on the surface of the cathode possess enough thermal energy that they can overcome the potential barrier on the surface of the cathode. The surfaces of thermal cathodes are chosen therefore with suitable surface layers that keep the energy that electrons require to escape as low as possible, this in order that high electron emission can be achieved.
A further possibility for producing electron emitting cathode surfaces is to apply a high electrical field force to a cold cathode, i.e. a cathode that has not been specially heated. Such cold electron-emitting cathode surfaces are called field emission surfaces. In order to achieve field emission currents of any significance, it is necessary to apply very high electrical field forces to the cathode surface. In order to keep the voltage applied to the cathode to as low a level as possible, and at the same time to achieve high electrical field forces locally, the cathode surfaces are usefully provided with finely structured peaks.
The flat screens e.g. in present-day laptop computers or portable television sets normally function as LCD (liquid crystal display) screens. Such LCD screens, however, allow only low switching rates with fast moving pictures, and in general the quality of color reproduction does not match that required of conventional tube-type screens.
The technology offered by field emission screens (FED or field emission display) overcomes the disadvantages encountered with LCD screens.
FED screens usually comprise of a conventional, but not curved, phosphor-screen with a mask. A plate-shaped cathode is situated a distance e.g. 0.2 mm from it and features a matrix of fine, sharp peaks. These peaks may carry or be subjected group-wise to high voltage current, as a result of which electrons are emitted because of the field effect. The emitted electrons are then accelerated and so activate the facing illuminating material on the phosphor screen.
An image element of an FED screen is usefully comprised of three points which are provided with a red, green or blue light-emitting material. Directed at each of these points on the cathode side are about one thousand micro-peaks which together supply such a high yield of field-effect electrons that the FED screen exhibits much lower power consumption than conventional tube-type screens for the same brightness.
Compared with the LCD screen, the FED screen offers the advantage of inertia-free control of each image point. Also, image quality is independent of the angle of viewing.
A known method of manufacturing cold emitting cathode surfaces is to microstructure the cathode surface using photo-lithographic techniques that have been used for a long time now in the production of semiconductor elements. This method involves first using photolithographic techniques to create a photo-sensitive mask on the cathode surface having a field with a rectangular or circular opening. In a second step the substrate area not protected by the mask is etched such that, after dissolution of the photo-sensitive mask, pyramid or conical shaped emitter peaks are produced.
A further possibility for manufacturing field emission surfaces is isotropic etching of a crystalline material such as e.g. Si, producing fine peaks that are coated e.g. with an electron-emitting material. Also, semiconductor materials such as Si can be structured by photo-lithographic methods and e.g. subsequently coated with an electron emitting material.
The U.S. Pat. No. 4,591,717 describes a photoelectric detector based on a field emission surface having a light sensitive layer with a plurality of peaks of electrical conductive material. The peaks are produced by anodic oxidation of a substrate surface, in which process pores lying perpendicular to the substrate surface are formed and metal is precipitated into the said pores in such a manner that metal peaks that project beyond the oxide layer are formed.
The European patent EP 0 351 110 describes a process for manufacturing cold cathode emitter surfaces in which an aluminum oxide surface is provided with a plurality of longitudinal pores lying essentially perpendicular to the main surface of the aluminum oxide layer. The pores are filled with an electron emitting material then at least a part of this aluminum oxide layer is removed, as a result of which a surface with exposed electron emitting peaks is produced and the peaks face each other.
The state-of-the-art field emission surfaces, manufactured by forming an oxide layer containing pores, depositing electron emitting material on the surface layer and in the pore cavities, and subsequently removing the layer containing the pores, always exhibit at most as many electron-emitting peaks as the number of pores in the oxide layer contributing to their manufacture.